The invention concerns a method of controlling the content of fat in milk. Whole milk is separated into skim milk and cream. Some of the cream is returned to the skim milk to create a standardized milk. The standardized milk is then employed to derive a parameter that dictates how much cream is to be added. The fat contents detected in the standardized milk are exploited to vary the amount of added cream. The density of the skim milk, however, is also measured at intervals and the results are stored. The density of the standardized milk is then measured and more or less cream is added until a prescribed difference between the skim milk and the standardized milk, corresponding to the desired fat content in the standardized milk, is obtained.
A method of this type is known from German OS 2 531 141. The densities of the skim milk and cream leaving a centrifuge are detected by two separate sensors. Some of each liquid flows through the sensors. The two currents subsequently rejoin the mainstream. How much cream to add to the skim milk in order to obtain a particular fat content in the standardized milk accordingly depends on the empirical difference between the densities of the two liquids.
Density, however, depends not only on fat content but also on such non-fat solids as proteins, sugar, minerals etc. and on temperature. Measuring the density of the skim milk and that of the standardized milk and calculating the difference between them is intended in this known process to eliminate all factors other than fat content such that the difference between the results will depend only on the difference between the fat content of the skim milk and that of the standardized milk. Since the same product is theoretically flowing through both sensors, it is assumed that establishing a difference in density that represents the desired content of fat in the standardized milk will also maintain that content constant.
Using two sensors, however, necessarily entails two basic sources of error. One source comprises manufacturing tolerances that lead to discrepancies between the results obtained from the two sensors even when the products flowing through them are equally dense. The other source is the difference in on-site fluidics that derives from the alignment between the two sensors, necessitating a pressure difference to ensure flow. When the products, as is common with milk, contain air, the air will lead to different density readings in the event of pressure differences, even though the fat content does not differ.
Still other errors can derive from variations in the fat content of the skim milk throughout an operating cycle. Such variations can occur for example as the result of reductions in the centrifuge's separating efficiency, of the presence of homogenized milk in the milk being skimmed, and of different percentages of air in the whole milk. The difference in density detected by the first and the second sensor will no longer ensure the desired fat content in the standardized milk.